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Article

Effects of Annealing Temperature on the CrystalStructure, Morphology, and Optical Properties ofPeroxo-Titanate Nanotubes Prepared byPeroxo-Titanium Complex Ion

Hyunsu Park 1 , Tomoyo Goto 1 , Sunghun Cho 1 , Soo Wohn Lee 2, Masato Kakihana 1,3

and Tohru Sekino 1,*1 Department of Advanced Hard Materials, The Institute of Scientific and Industrial Research (ISIR),

Osaka University, Osaka 567-0047, Japan; [email protected] (H.P.);[email protected] (T.G.); [email protected] (S.C.); [email protected] (M.K.)

2 Department of Environmental and Bio-chemical Engineering, Sun Moon University, Asan 31460, Korea;[email protected]

3 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan* Correspondence: [email protected]; Tel.: +81-(0)-6-6879-8435

Received: 13 June 2020; Accepted: 6 July 2020; Published: 8 July 2020��

Abstract: This study addresses the effects of annealing temperatures (up to 500 ◦C) on the crystalstructure, morphology, and optical properties of peroxo groups (–O–O–) containing titanate nanotubes(PTNTs). PTNTs, which possess a unique tubular morphology of layered-compound-like hydrogentitanate structure (approximately 10 nm in diameter), were synthesized using peroxo-titanium(Ti–O–O) complex ions as a precursor under very mild conditions—temperature of 100 ◦C and alkaliconcentration of 1.5 M—in the precursor solution. The nanotubular structure was dismantled byannealing and a nanoplate-like structure within the range of 20–50 nm in width and 100–300 nm inlength was formed at 500 ◦C via a nanosheet structure by decreasing the specific surface area. Hydrogentitanate-based structures of the as-synthesized PTNTs transformed directly into anatase-type TiO2 at atemperature above 360 ◦C due to dehydration and phase transition. The final product, anatase-basedtitania nanoplate, was partially hydrogen titanate crystal in nature, in which hydroxyl (–OH) bondsexist in their interlayers. Therefore, the use of Ti–O–O complex ions contributes to the improvedthermal stability of hydrogen titanate nanotubes. These results show a simple and environmentallyfriendly method that is useful for the synthesis of functional nanomaterials for applications invarious fields.

Keywords: titanate nanotubes; titanate nanosheets; titanate nanoplates; peroxo-titanium complexion; annealing

1. Introduction

One-dimensional nanostructures, such as nanowires [1], nanobelts [2], nanorods [3],and nanotubes [4] which have received great attention due to their intriguing nanostructure andexcellent properties [5]. Titania and titanate materials are nanostructures and they have excellentproperties. One-dimensional nanostructured titania and titanate materials have attracted considerableattention and have been widely investigated in various fields such as environmental purification,photocatalysis, self-cleaning coatings, gas-sensor materials, electrode materials for dye-sensitized solarcells, as well as electron-transfer-layer (ETL) for perovskite photovoltaic cells [6–10]. This is due totheir high redox potential, chemical stability, inexpensiveness, and non-toxicity [11]. The wide variety

Nanomaterials 2020, 10, 1331; doi:10.3390/nano10071331 www.mdpi.com/journal/nanomaterials

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Nanomaterials 2020, 10, 1331 2 of 17

of their functional properties can be improved by the control of the nanostructure of titania and titanate.Nanotube structures [12], in particular, have received a lot of attention due to their large specific surfaceareas as well as internal and external surfaces, and their high porosity structures [13], which increasethe number of potential active sites for a given reaction [14] that improves the catalytic properties andelectricity conversion effects [15].

There are many routes for synthesizing titania/titanate nanotubes (TNTs), which includeanodizing [16], sol-gel template synthesis [17], hydrothermal [4], and solution chemical synthesismethods [18] that are based on alkali treatment. Among these methods, the chemical synthesis route isrelatively simple and facile. Although TNTs with an average diameter of 10 nm and a high specificsurface area could be synthesized using this method, this process requires a temperature of 115 ◦C andduration above 24 h, as well as a high alkaline concentration above 10 M NaOH of the synthesis solution.However, the NaOH is classified as a toxic chemical by the Toxic Use Reduction Act. When used inlarge quantities, it poses significant chemical hazards to the environment or our health. Many groupshave tried to modify this process and analyze the structure of TNTs, and they have reported thatTNTs are composed of a layered titanate structure. They proposed that synthetic mechanism shouldbe the dissolution/recrystallization process [19]. In many cases, titanate compounds such as porouscrystalline titanate [20] or titanium alkoxides were used as starting materials for the synthesis ofnanostructured titanates.

As mentioned earlier, the use of these raw materials requires severe synthesis conditions andhigh energy, which is due to the use of a large number of alkaline species for dissolving raw materialscontaining Ti. From the perspective of green chemistry, the use of titanium chelate complexes havealso been reported [21–23]. This study introduces the novel bottom-up synthesis process for tubularstructured titanate at a relatively low alkaline concentration of 1.5 M, using peroxo-titanium (Ti–O–O)complex ion as a precursor (named as PTNTs, peroxo-titanate nanotubes). In this way, this PTNTs canbe also synthesized at a low temperature of approximately 100 ◦C using a novel bottom-up synthesismethod, which is an environmentally-friendly process PTNTs possess a special peroxo-titanium bond(Ti–O–O) in the crystals as well as on the surface, which may enhance light absorption by reducing theoptical band gap energy [24,25]. However, to apply this material to various engineering applications,it is very important to evaluate its thermal properties. For example, when applied as electrodes todye-sensitized solar cells or chemical sensor devices, thermal treatment is required to make strongand stable oxide films/coatings, remove the binder, and for adhesion with substrates. Moreover,many reports demonstrated the TiOx conducting layer annealed at high temperature so the electronscan be transferred through the valence band of the TiOx [26]. However, the heating of nanostructuredmaterials often affects the morphological change and the crystal structure due to sintering and chemicalreactions [27,28]. In gas sensor applications as oxide semiconductors, the operating temperature fordetecting the target gas is usually high and related to a variety of properties of the material such as thecrystal structure, the particle size, and the energy band structure. Therefore, the evaluation of the effectof heating is essential for effectively setting the operating temperature and effective detection [29].In the case of nanostructured titania/titanate synthesized at low temperature as described above,detailed understanding and clarifications on the relationship between the crystallographic propertiesinduced by varying the operating temperature and the effects on functionality need to be considered.

Various studies have reported the structural changes of TNTs effected by thermal treatment [30–34].Zhang et al. [34] reported the formation of the anatase phase by heating at a temperature above 500 ◦Cand the change in nanotubular morphology to a spherical shape. We reported that the synthesis ofTNTs following Kasuga’s method preserved the nanotubular morphology and maintained its highsurface area, but it was transformed to an anatase-type TiO2 at 450 ◦C, while a sudden decrease inthe surface area and morphological change to a spherical shape was confirmed by further heatingabove 450 ◦C [33]. PTNTs contain peroxo groups (–O–O–) that modify the chemical bonding natureof Ti–O and the resultant crystalline structures. Therefore, PTNTs are expected to show differentthermal stability, as well as crystallographic and morphological changes compared to the commonly


known titania/titanate nanotubes because of its unique structure with the peroxo group. In addition,understanding the thermal properties of TNTs is an important factor for designing TNTs’ functionsand applications. Etacheri et al. [11] reported the high-temperature stability of oxygen-rich titaniasynthesized by a peroxo-titania complex. However, there are few studies on the effects of heat treatmenton the morphological and crystallographic characteristics, as well as physical/chemical properties ofperoxo-titanium containing nanostructured titanate, especially nanotubular titanate (thus PTNTs). It isnecessary to develop an effective method for the synthesis of titanate nanostructures with good visiblelight responsibility even after the heating at high temperatures.

The purpose of this study is thus to clarify the relationships between the heat treatment of thecrystalline structures and the morphology of the titanate nanotubes containing PTNTs that was preparedfrom peroxo-titanium complex ions at a temperature as low as 100 ◦C. In situ high-temperature X-raydiffraction was used to observe the crystallographic changes induced by the thermal treatment ofPTNTs. The effect of annealing temperature on the morphological and optical characteristics of PTNTsis discussed using physicochemical investigations.

2. Materials and Methods

2.1. Synthesis of Materials

As a precursor, peroxo-titanium complex ion [35] was synthesized by the following method:Initial 0.63 g of TiH2 (> 99%, Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan) was dissolved in62.46 mL of mixed solution with a pH of 10 containing H2O2 (30%, FUJIFILM Wako Pure ChemicalCorporation, Osaka, Japan) and NaOH (97%, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan)aqueous solution for 2 h at 10 ◦C. The pH of the mixed solution was controlled by gradually adding10 M of concentrated NaOH solution to the hydrogen peroxide solution. Consequently, the Naconcentration in the precursor solution was maintained at 1.5 mol/L (1.5 M) and the last step is to addalkaline (Na) to synthesize nanotubular titanate. It should be noted that the necessary amount of Narequired for the synthesis of TNTs in this study was much less compared to the commonly knownchemical synthesis process, which is typically 10 M [18]. To synthesize peroxo-titanate nanotubes,the prepared peroxo-titanium complex ion solution was put in a polytetrafluoroethylene bottleequipped with a reflux condenser and it was placed in an oil-bath. It was heated at 100 ◦C for 12 hwith and it was stirred at string speed of 200 rpm. Afterward, the precipitates were treated using a 5 MHCl (30%, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) solution until the pH became 5,and it was washed using distilled water and a vacuum pump (MDA-020C, ULVAC, Inc.,Kanagawa, Japan) until the ion conductivity of the filtered solution became less than 5 µS/cm.The product was dried using a freeze dryer (EYELA FDU-2200, TOKYO RIKAKIKAI CO, LTD.,Tokyo, Japan) and was labeled as PTNTs. Afterward, to investigate the effect of heating on PTNTs,the samples were thermally treated at 200, 300, 400, and 500 ◦C using a furnace. All samples wereheated under ambient conditions at 10 ◦C/min and kept at a given temperature for 1 h.

2.2. Characterization

The thermal properties of the PTNTs were studied using thermogravimetric differential thermalanalysis (TG-DTA, TG8120, RIGAKU, Tokyo, Japan). Approximately 10 mg of the sample was weighedon a Pt pan and the α-alumina powder was used as the reference material. Scans between 25 and600 ◦C were carried out at a heating rate of 10 ◦C/min under ambient conditions. The temperature of theinstrument was calibrated by using purity standard metallic materials (In, Pb and Au). The morphologyand particle size were observed using field-emission scanning electron microscopy (FE-SEM, SU-9000,Hitachi High-Technologies Corporation, Tokyo, Japan) with 0.4 nm resolution at 30 kV accelerationvoltage. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method basedon the adsorption isotherm at a P/P0 range of 0.1–0.3 using an N2 adsorption-desorption instrument(NOVA 4200e, Quantachrome Instruments, Boynton Beach, FL, USA). The crystalline phase was


identified using X-ray diffraction (XRD, D8 ADVANCE, Bruker AXS Co. Ltd., Karlsruhe, Germany),and the diffraction patterns of each sample were collected using a vertical goniometer equipped witha high-temperature reaction chamber (XRK-900, Anton Paar, Graz, Austria), operating in the Braggconfiguration using Cu Kα radiation (λ = 1.54056 Å) from 5 to 85◦ at a scanning rate of 0.02◦. In-situXRD studies were recorded during the annealing process, heating occurred from 25 to 560 ◦C at arate of 10 ◦C/min. From the results of the XRD analysis, the lattice parameters of the samples wereevaluated using Equation (1).

1d2

=h2

a2+

k2

b2+

l2

c2(1)

where, d is the interplanar spacing and h, k, l are Miller indices. a, b, and c are lattice constant of crystalphase. The anatase crystallite size of samples was evaluated using Equation (2).

D =0.94λβcosθ

(2)

where D is the crystalline size of materials, λ is the wavelength of X-ray, β is the broadening of thediffraction line measured at half of its maximum intensity in radians, and θ is the angle of diffraction.The shape factor of 0.94 was used by assuming a sphere shape in this study. Transmission electronmicroscopy (TEM, JEOL-2100, JEOL, Tokyo, Japan) was performed at an acceleration voltage of200 kV to further characterize individual nanostructures of titanate or titania. The lattice spacing,fast Fourier transform (FFT), and the phase interpretation were investigated using the Gatan DigitalMicrograph software [36] (Gatan Inc., Pleasanton, CA, USA). Fourier transform infrared spectrometer(FT-IR, FT/IR4100, JASCO, Tokyo, Japan) spectra were obtained within the range of 4000–500 cm−1

region at a resolution of 4 cm−1. The samples were measured using the KBr pellet method in thetransmission mode. Raman spectra were acquired with a Raman micro-spectrometer (HR 800, HORIBA,Kyoto, Japan) using an Ar ion laser (514.5 nm). The spectra were collected in the range of 1000–100cm−1 with a resolution of 1 cm−1. The reflectivity of the prepared powder was evaluated using the solidsample measurement mode in the ultraviolet-visible (UV-Vis) machine. Diffuse reflectance spectroscopy(DRS) (V-650, JASCO Co., Tokyo, Japan) and optical band gap energy were measured using the Tauc-plotmethod using Kubelka–Munk transformation. The powders were uniformly pressed in a powderholder and placed in the sample holder on an integrated sphere for the reflectance measurements.X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)spectra of the Ti K-edge were measured in an ionization chamber in transmission mode on the KyushuUniversity Beamline (BL06) of the Kyushu Synchrotron Light Research Centre (SAGA-LS; Tosu, Japan).Pellet samples for Ti K-edge XANES and EXAFS measurements were prepared as mixtures with boronnitride. Anatase (TiO2, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) was used as areference sample. Ti K-edge spectra were collected over a photon energy range of 4635.2–5938.6 eV,and the spectra were analyzed using ATHENA software [37] (Version 0.9.26, Ravel and Newville 2005,).

3. Results and Discussion

3.1. Thermal Properties of Materials

The powdered products were successfully synthesized following the chemical processing whereperoxo-titanium complex ion solution is used as a precursor for the synthesis of nanostructuredtitania. The powder was yellow, and the optical bandgap energy was greatly reduced to 2.44 eV whencompared with the white color of pure TNTs with an optical band gap of 3.34 eV. The yellow-coloredTiO2-based compounds have been synthesized by anion doping such as N [38] and cation doping likeNb [39]. However, the peroxo-titanium complex ion precursor contains only H, O, Na and Ti in solution,and thus the bottom-up process in this study does not include any doping elements like N and Nbthat introduce formation of impurity levels among the band gap in TNTs. Therefore, this yellow coloris considered to be caused by the reduced band gap of titanate without any doping. In addition,


this implies that the present processing method that makes use of precursor complex ions and followingthe chemical route might be a facile but promising method for the production of functionalized oxideswith low-dimensional nanostructures.

The thermal properties of the prepared PTNTs in the temperature range of 25–600 ◦C werecharacterized by TG and DTA, as shown in Figure 1. The total weight loss was approximately 17.2%,and the significant weight loss was observed in the temperature range of 25–143 ◦C. Within this range,10.3% of weight loss was estimated to be due to the desorption of molecular H2O such as dissociatedH2O, physisorbed H2O, and chemisorbed H2O [32]. This weight loss can also be attributed to thestructural water loss of the titanium oxide structure including the Ti–OH bond [40,41]. An endothermicpeak was observed at approximately 57 ◦C in the DTA curve due to water evaporation of physicallyadsorbed water on the PNTs. Subsequently, a continuous weight loss of 6.9% was obtained at 600 ◦C,which may also be assigned to structural water loss, resulting in the transformation of the crystalstructure [32]. An exothermic peak is observed in the DTA graph due to the transformation of a crystalstructure at approximately 257 ◦C.

Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 17

The thermal properties of the prepared PTNTs in the temperature range of 25–600 °C were

characterized by TG and DTA, as shown in Figure 1. The total weight loss was approximately 17.2 %,

and the significant weight loss was observed in the temperature range of 25–143 °C. Within this range,

10.3% of weight loss was estimated to be due to the desorption of molecular H2O such as dissociated

H2O, physisorbed H2O, and chemisorbed H2O [32]. This weight loss can also be attributed to the

structural water loss of the titanium oxide structure including the Ti–OH bond [40,41]. An

endothermic peak was observed at approximately 57 °C in the DTA curve due to water evaporation

of physically adsorbed water on the PNTs. Subsequently, a continuous weight loss of 6.9% was

obtained at 600 °C, which may also be assigned to structural water loss, resulting in the

transformation of the crystal structure [32]. An exothermic peak is observed in the DTA graph due to

the transformation of a crystal structure at approximately 257 °C.

Figure 1. Thermogravimetric (TG) and differential thermal analysis (DTA) curves of as-synthesized

peroxo-titanate nanotubes (PTNTs) heated at 10 °C/min in an air atmosphere.

3.2. Morphology of Materials

The SEM images of prepared samples are shown in Figure 2. The as-synthesized samples had

tubular structures, as shown in Figure 2a. The average diameter of tubular structures was found to

be approximately 10–20 nm, and after heating at 200 °C, distorted tubular structures were often

observed (Figure 2b). At the heating temperature of 300 °C, sheet-like structures were frequently seen,

as shown in Figure 2c. The sheet-like structures seem to stack on each other, and these stacked sheet-

like structures were still observed in the sample that was heated up to 400 °C (Figure 2 d). Further

annealing at 500 °C resulted in the formation of a plate or rod structures, as shown in Figure 2e. This

was observed not only in the specific part among the agglomerates in the samples but also in the

whole part of each sample.

Figure 1. Thermogravimetric (TG) and differential thermal analysis (DTA) curves of as-synthesizedperoxo-titanate nanotubes (PTNTs) heated at 10 ◦C/min in an air atmosphere.

3.2. Morphology of Materials

The SEM images of prepared samples are shown in Figure 2. The as-synthesized sampleshad tubular structures, as shown in Figure 2a. The average diameter of tubular structures wasfound to be approximately 10–20 nm, and after heating at 200 ◦C, distorted tubular structureswere often observed (Figure 2b). At the heating temperature of 300 ◦C, sheet-like structures werefrequently seen, as shown in Figure 2c. The sheet-like structures seem to stack on each other,and these stacked sheet-like structures were still observed in the sample that was heated up to400 ◦C (Figure 2d). Further annealing at 500 ◦C resulted in the formation of a plate or rod structures,as shown in Figure 2e. This was observed not only in the specific part among the agglomerates in thesamples but also in the whole part of each sample.



Figure 2. Scanning electron microscopy (SEM) images of (a) as-synthesized PTNTs and (b–e) annealed

PTNTs at 200, 300, 400 and 500 °C for 1 h, respectively. (f) Schematic description of the annealing

temperature effect on the morphologies of PTNTs.

Figure 2f shows the schematic diagram of the morphology change caused by the increase in

annealing temperatures, which mainly represents the typical structures at each temperature, as

shown in dotted boxes in each SEM image (Figure 2a–e). The inset in Figure 2f represents the expected

cross-section of each structure. The as-synthesized PTNTs had a tubular structure. This tubular

structure seems to be “dismantled” above 200 °C of annealing, and at approximately 200 °C, the end

part of the tube seems to re-open by forming a cleavage that seems to extend further. At 300 and 400

°C, “stacked bamboo leaf-like nanosheets” were frequently observed which might be formed by the

re-opening of PTNTs. Although the structure, composition, and formation mechanism of titania

nanotubes synthesized by chemical treatment methods are still under review [42], a nanotube

structure originated from the characteristics of the layered hydrogen titanate crystal [13]. Thus, the

TNTs are considered to be formed by the scrolling of titanate nanosheets to make a multi-wall titanate

nanotube. Considering these facts, the formation of stacked leaf-like nanosheets by the re-opening of

nanotubes, as shown in Figure 2c and d as well as the illustration in Figure 2e, may be feasible. From

these perspectives, the phenomenon of the deformed tubular structure may imply a return to a sheet

structure as a result of an increase in the annealing temperature. The annealing temperature at 300

°C causes the tubular structures to change into stacked sheet-like structures, and the sheet structure

was assumed to show unstable surface energy due to its large surface area. However, the layer

structure was gradually distorted due to the removal of water molecules from PTNTs by the

annealing process as shown by the TG-DTA results (Figure 1). Therefore, it would be built up together

to minimize its surface energy at a higher temperature (approximately 400 °C). As a result,

transformation into a plate-like or rod-like structure consisting of bamboo leaf-like nanosheets stacks

with a long aspect ratio was formed at a higher temperature that was up to 500 °C as surrounded by

square dotted frames in Figure 2e as well as illustrated in Figure 2f. The fundamental mechanism of

how the heating or thermal activation would make such a structural variation in the PTNTs has not

been clarified yet, and thus the further detailed investigations are required through various in situ

analysis using transmission/scanning transmission electron microscopy (STEM/TEM), thermal

analysis coupled with mass spectroscopy and Fourier transform infrared (FT-IR) spectroscopy etc.

However, we inferred that the decomposition of peroxo functional groups (–O–O–) and/or release of

H2O might contribute to the systematic structure degradation of layered titanate nanotubes because

such structure degradation has not been reported yet for the pure titania/titanate nanotubes.

Figure 3 shows the specific surface area (SSA) at various annealing temperatures. SSA decreased

linearly (R-squared value = 0.9907) as the annealing temperature increased. In this study, the

annealing temperature affected the morphology of the structure as observed in Figure 2. In this study,

Figure 2. Scanning electron microscopy (SEM) images of (a) as-synthesized PTNTs and (b–e)annealed PTNTs at 200, 300, 400 and 500 ◦C for 1 h, respectively. (f) Schematic description ofthe annealing temperature effect on the morphologies of PTNTs.

Figure 2f shows the schematic diagram of the morphology change caused by the increase inannealing temperatures, which mainly represents the typical structures at each temperature, as shownin dotted boxes in each SEM image (Figure 2a–e). The inset in Figure 2f represents the expectedcross-section of each structure. The as-synthesized PTNTs had a tubular structure. This tubularstructure seems to be “dismantled” above 200 ◦C of annealing, and at approximately 200 ◦C, the endpart of the tube seems to re-open by forming a cleavage that seems to extend further. At 300 and400 ◦C, “stacked bamboo leaf-like nanosheets” were frequently observed which might be formed bythe re-opening of PTNTs. Although the structure, composition, and formation mechanism of titaniananotubes synthesized by chemical treatment methods are still under review [42], a nanotube structureoriginated from the characteristics of the layered hydrogen titanate crystal [13]. Thus, the TNTs areconsidered to be formed by the scrolling of titanate nanosheets to make a multi-wall titanate nanotube.Considering these facts, the formation of stacked leaf-like nanosheets by the re-opening of nanotubes,as shown in Figure 2c,d as well as the illustration in Figure 2e, may be feasible. From these perspectives,the phenomenon of the deformed tubular structure may imply a return to a sheet structure as a resultof an increase in the annealing temperature. The annealing temperature at 300 ◦C causes the tubularstructures to change into stacked sheet-like structures, and the sheet structure was assumed to showunstable surface energy due to its large surface area. However, the layer structure was graduallydistorted due to the removal of water molecules from PTNTs by the annealing process as shown by theTG-DTA results (Figure 1). Therefore, it would be built up together to minimize its surface energy at ahigher temperature (approximately 400 ◦C). As a result, transformation into a plate-like or rod-likestructure consisting of bamboo leaf-like nanosheets stacks with a long aspect ratio was formed at ahigher temperature that was up to 500 ◦C as surrounded by square dotted frames in Figure 2e as wellas illustrated in Figure 2f. The fundamental mechanism of how the heating or thermal activationwould make such a structural variation in the PTNTs has not been clarified yet, and thus the furtherdetailed investigations are required through various in situ analysis using transmission/scanningtransmission electron microscopy (STEM/TEM), thermal analysis coupled with mass spectroscopy andFourier transform infrared (FT-IR) spectroscopy etc. However, we inferred that the decomposition ofperoxo functional groups (–O–O–) and/or release of H2O might contribute to the systematic structuredegradation of layered titanate nanotubes because such structure degradation has not been reportedyet for the pure titania/titanate nanotubes.


Figure 3 shows the specific surface area (SSA) at various annealing temperatures. SSA decreasedlinearly (R-squared value = 0.9907) as the annealing temperature increased. In this study,the annealing temperature affected the morphology of the structure as observed in Figure 2.In this study, the surface area decreased, and it is expected that the dehydration of the surfaceof the structures caused agglomeration between each structural unit (i.e., individual nanotubes) andthis reduces the specific surface area because the morphology and crystal structure were affected athigher temperatures [34], which was confirmed by the findings from the TG-DTA analysis shown inFigure 1.


the surface area decreased, and it is expected that the dehydration of the surface of the structures

caused agglomeration between each structural unit (i.e., individual nanotubes) and this reduces the

specific surface area because the morphology and crystal structure were affected at higher

temperatures [34], which was confirmed by the findings from the TG-DTA analysis shown in Figure

1.

Figure 3. Variation of the specific surface areas of PTNTs annealed at 200, 300, 400, and 500 °C for 1 h,

respectively.

3.3. Crystallographic Characteristics of Materials

The crystalline phase variation of the PTNTs caused by heating was analyzed by in situ high-

temperature X-ray diffraction, and the results are shown in Figure 4a. The as-synthesized PTNTs

show a typical hydrogen titanate phase [43] (Powder Diffraction File (PDF) Card# 00-047-0124) with

an orthorhombic structure. Titanate nanotube (H2Ti2O5·H2O) is considered to be formed by scrolling

layers of titanate nanosheets and thus forming a layered-structure that corresponds to the 200

reflections of TNTs. The diffraction peaks related to the c-axis (e.g., 501, 002) were much weaker than

the peaks at 200, 110, 310, 020 [44]. As the annealing temperature increases, the crystal phase is

converted from hydrogen titanate to an anatase-type (PDF Card# 00-021-1272) with tetragonal

structure. The lattice constants of reported H2Ti2O5·H2O [43] were: a0 = 18.03 Å , b0 = 3.784 Å , and c0 =

2.998 Å (PDF Card# 00-047-0124), and the lattice constants of the reported anatase were: a0 = 3.785 Å ,

b0 = 3.785 Å , and c0 = 9.514 Å (PDF Card# 00-021-1272). The lattice constants of anatase obtained after

heating to 500 °C were: a0 = 3.764 Å , b0 = 3.764 Å , and c0 = 9.483 Å . The lattice constants were calculated

using Equation (1) and the values of the as-synthesized PTNTs were a0=19.27 Å , b0=3.743 Å and

c0=2.976 Å . Among the 3 parameters, the b0 and c0 values agreed with those of the base hydrogen

titanate structure. However, the a0 value, which relates to the interlayer spacing of the titanate

structure increased by 6.9 %, which is more than that for the common hydrogen titanate. The large

increase in the a-axis may be due to the enlargement of interlayer distance at the 200 plane. Sugita et

al. [45] reported an increase in the interplanar spacing of titanate when the radius of the Li+ ions was

greater than that of H+ ions located between the hydrogen titanate layers. Kong et al. [24] obtained

similar results when a peroxo-titanium bond (Ti–O–O–H) was present between the layers of

hydrogen titanate crystals.

The variations of each peak position (2), and the intensity assigned to 110 diffraction peaks of

PTNTs (hydrogen titanate), the 101 diffraction peak of anatase phases, and the crystallite size with

the annealing temperature of samples are displayed in Figure 4b. The 2 angle of the diffraction peak

assigned to 110 of hydrogen titanate crystal was maintained at 25.00° ± 0.12° till the heating

Figure 3. Variation of the specific surface areas of PTNTs annealed at 200, 300, 400, and 500 ◦C for 1 h, respectively.

3.3. Crystallographic Characteristics of Materials

The crystalline phase variation of the PTNTs caused by heating was analyzed by in situhigh-temperature X-ray diffraction, and the results are shown in Figure 4a. The as-synthesizedPTNTs show a typical hydrogen titanate phase [43] (Powder Diffraction File (PDF) Card# 00-047-0124)with an orthorhombic structure. Titanate nanotube (H2Ti2O5·H2O) is considered to be formed byscrolling layers of titanate nanosheets and thus forming a layered-structure that corresponds to the200 reflections of TNTs. The diffraction peaks related to the c-axis (e.g., 501, 002) were much weakerthan the peaks at 200, 110, 310, 020 [44]. As the annealing temperature increases, the crystal phase isconverted from hydrogen titanate to an anatase-type (PDF Card# 00-021-1272) with tetragonal structure.The lattice constants of reported H2Ti2O5·H2O [43] were: a0 = 18.03 Å, b0 = 3.784 Å, and c0 = 2.998 Å(PDF Card# 00-047-0124), and the lattice constants of the reported anatase were: a0 = 3.785 Å,b0 = 3.785 Å, and c0 = 9.514 Å (PDF Card# 00-021-1272). The lattice constants of anatase obtained afterheating to 500 ◦C were: a0 = 3.764 Å, b0 = 3.764 Å, and c0 = 9.483 Å. The lattice constants were calculatedusing Equation (1) and the values of the as-synthesized PTNTs were a0 = 19.27 Å, b0 = 3.743 Å andc0 = 2.976 Å. Among the 3 parameters, the b0 and c0 values agreed with those of the base hydrogentitanate structure. However, the a0 value, which relates to the interlayer spacing of the titanate structureincreased by 6.9%, which is more than that for the common hydrogen titanate. The large increase in thea-axis may be due to the enlargement of interlayer distance at the 200 plane. Sugita et al. [45] reportedan increase in the interplanar spacing of titanate when the radius of the Li+ ions was greater than thatof H+ ions located between the hydrogen titanate layers. Kong et al. [24] obtained similar results whena peroxo-titanium bond (Ti–O–O–H) was present between the layers of hydrogen titanate crystals.



temperature was up to 350 °C. Then, the peak position quickly shifted to a higher angle at the

subsequent temperature range, and the 2 value was 25.28° ± 0.12° which is the similar peak position

of 101 in anatase TiO2 phase (calculated as 25.281° from the d-value). At the same time, as the

temperature increased, the intensity of peak maintained a low value till the temperature was 350 °C

and it linearly increased to 460 °C from the subsequent annealing temperature where it maintained

its maximum value. The crystallite size of the anatase phase was calculated at 350 °C to be

approximately 13 nm, which then increased to 19 nm as the annealing temperature increased up to

460 °C were maintained its highest size.

Figure 4. (a) In situ X-ray diffraction (XRD) patterns of the PTNTs during an annealing temperature.

(b) Annealing temperature effect on the variations of 2 and Peak intensity assigned to the 110

hydrogen titanate and 101 anatase reflections, and the crystal size of the anatase phase. (c) Annealing

temperature effect on the variations of 2 and peak intensity assigned to the 200 hydrogen titanate

reflection and d-value derived from 2. (d) Illustration for variation of interlayer distance by annealing.

The change in the peak was also observed at the diffraction peak according to the 200 of

hydrogen titanate. Figure 4c shows the variations in the peak position (2, intensity, and d-value of

the 200 plane depending on the annealing temperature. No noticeable change was observed at the

diffraction peak of 200 until a temperature change of 100 °C was attained. As the annealing

temperature increases, the peak position shifted to a higher 2 angle with a decrease in the peak

intensity. As earlier mentioned, this 200 peak was assigned to the interlayer of hydrogen titanate, for

which the interlayer distance of PTNTs could be said to decrease with the heating, as shown in Figure

4d. From the TG-DTA analysis in Figure 1, the heating at low-temperature up to approximately 120

°C, in which the major phenomenon was the removal of physically-adsorbed water from the surface,

did not affect the structural characteristics of PTNTs. Furthermore, subsequent water loss could be

attributed to the interlayer water loss as the peak shifted to a high angle, which indicates a reduction

in the interlayer distance, as observed in XRD results. Our results show that the interlayer distance

of approximately 0.9 nm in the as-synthesized PTNTs was maintained at 120 °C, followed by a rapid

decrease in the distance within a temperature range of up to 200 °C. Then, as the annealing

temperature further increases, the interplanar distance decreases linearly which results in a reduction

of d = 0.7 nm at 560 °C.

3.4. Transmission Electron Microscopy

Figure 4. (a) In situ X-ray diffraction (XRD) patterns of the PTNTs during an annealing temperature.(b) Annealing temperature effect on the variations of 2θ and Peak intensity assigned to the 110 hydrogentitanate and 101 anatase reflections, and the crystal size of the anatase phase. (c) Annealing temperatureeffect on the variations of 2θ and peak intensity assigned to the 200 hydrogen titanate reflection andd-value derived from 2θ. (d) Illustration for variation of interlayer distance by annealing.

The variations of each peak position (2θ), and the intensity assigned to 110 diffraction peaks ofPTNTs (hydrogen titanate), the 101 diffraction peak of anatase phases, and the crystallite size withthe annealing temperature of samples are displayed in Figure 4b. The 2θ angle of the diffractionpeak assigned to 110 of hydrogen titanate crystal was maintained at 25.00◦ ± 0.12◦ till the heatingtemperature was up to 350 ◦C. Then, the peak position quickly shifted to a higher angle at thesubsequent temperature range, and the 2θ value was 25.28◦ ± 0.12◦ which is the similar peak position of101 in anatase TiO2 phase (calculated as 25.281◦ from the d-value). At the same time, as the temperatureincreased, the intensity of peak maintained a low value till the temperature was 350 ◦C and it linearlyincreased to 460 ◦C from the subsequent annealing temperature where it maintained its maximumvalue. The crystallite size of the anatase phase was calculated at 350 ◦C to be approximately 13 nm,which then increased to 19 nm as the annealing temperature increased up to 460 ◦C were maintainedits highest size.

The change in the peak was also observed at the diffraction peak according to the 200 of hydrogentitanate. Figure 4c shows the variations in the peak position (2θ, intensity, and d-value of the 200 planedepending on the annealing temperature. No noticeable change was observed at the diffraction peakof 200 until a temperature change of 100 ◦C was attained. As the annealing temperature increases,the peak position shifted to a higher 2θ angle with a decrease in the peak intensity. As earlier mentioned,this 200 peak was assigned to the interlayer of hydrogen titanate, for which the interlayer distanceof PTNTs could be said to decrease with the heating, as shown in Figure 4d. From the TG-DTAanalysis in Figure 1, the heating at low-temperature up to approximately 120 ◦C, in which the majorphenomenon was the removal of physically-adsorbed water from the surface, did not affect thestructural characteristics of PTNTs. Furthermore, subsequent water loss could be attributed to theinterlayer water loss as the peak shifted to a high angle, which indicates a reduction in the interlayerdistance, as observed in XRD results. Our results show that the interlayer distance of approximately


0.9 nm in the as-synthesized PTNTs was maintained at 120 ◦C, followed by a rapid decrease in thedistance within a temperature range of up to 200 ◦C. Then, as the annealing temperature furtherincreases, the interplanar distance decreases linearly which results in a reduction of d = 0.7 nm at 560 ◦C.

3.4. Transmission Electron Microscopy

To confirm the interlayer distance and distribution of the crystallites of PTNTs, high-resolution(HR) TEM observation was performed for the samples obtained at different annealing temperatures,as shown in Figure 5. All the samples show a periodic lattice image that corresponds to the 200plane of layered hydrogen titanate, which was also observed from XRD analysis results (Figure 4).The interlayer distance was calculated to be 0.99 nm for the as-synthesized PTNTs (Figure 5a), and itclearly decreased with an increase in annealing temperature to 0.73 nm for PTNTs annealed at 500 ◦C(Figure 5e). The interlayer spacing obtained is summarized in Figure 5f, and the observed results wereconsistent with that calculated from the XRD analysis (d-spacing in Figure 4c). Their results werecompared and replotted as Figure S2, which clearly exhibited similar tendency.


To confirm the interlayer distance and distribution of the crystallites of PTNTs, high-resolution

(HR) TEM observation was performed for the samples obtained at different annealing temperatures,

as shown in Figure 5. All the samples show a periodic lattice image that corresponds to the 200 plane

of layered hydrogen titanate, which was also observed from XRD analysis results (Figure 4). The

interlayer distance was calculated to be 0.99 nm for the as-synthesized PTNTs (Figure 5a), and it

clearly decreased with an increase in annealing temperature to 0.73 nm for PTNTs annealed at 500 °C

(Figure 5e). The interlayer spacing obtained is summarized in Figure 5f, and the observed results

were consistent with that calculated from the XRD analysis (d-spacing in Figure 4c). Their results

were compared and replotted as Figure S2, which clearly exhibited similar tendency.

Figure 5. Transmission electron microscopy (TEM) images of (a) as-synthesized PTNTs and (b–e)

annealed PTNTs at 200, 300, 400, and 500 °C for 1 h, respectively. (f) Variation in the interlayer distance

for annealing temperatures taken from 30 zones, respectively.

Figure 6 shows the HR-TEM images and the FFT diffraction pattern of the annealed PTNTs at

400 and 500 °C. The d-values of the lattice fringes originated from diffraction spots which were

calculated from the FFT analysis of the TEM images; they were compared with the PDF card

information for hydrogen titanate (# 00-047-0124) and anatase (# 00-021-1272). In the case of the

PTNTs sample annealed at 400 °C, as shown in Figure 6a, the calculated diffraction spots for one

region was estimated to be the 200 plane of the hydrogen titanate with d-value of 0.7 nm. On the other

hand, a lattice fringe with a d-value of 0.37 nm was also observed in the same sample (enlarged image

and an FFT pattern in Figure 6a), which corresponds to the 101 plane of the anatase, in accordance

with XRD results, as shown in Figure 4. As was evident from the XRD analysis results, the annealed

PTNTs at 400 °C possess low crystallinity; the lattice fringes and diffraction spots assigned to only

the main 101 plane of anatase were expected to be observable, while other spots (e.g., corresponding

to 011, 112) are not expected to be observed.

At an annealing temperature of 500 °C, as shown in Figure 6b, two kinds of crystalline phases

were also identified. The FFT diffraction spots were estimated as (011), (101), and (112) planes of the

anatase as well as (200) plane of the hydrogen titanate. The enlarged image clearly shows the lattice

fringes of 0.65, 0.37, 0.37, and 0.24 nm, which are assigned to 200 of hydrogen titanate and 101, 011,

and 112 of anatase, respectively. These results are consistent with the results of the in situ XRD as

mentioned earlier in Figure 4. As earlier discussed, the sample annealed at 500 °C exhibited a rod-

like morphology with a width of 20–50 nm (see square dotted frames in Figure 2e). This rod-like

PTNTs contained the two crystalline structures of hydrogen titanate and anatase. This analysis

reveals that the 101 plane of anatase is parallel to the 200 plane of hydrogen titanate, and thus it is

believed that the crystallites of anatase were converted directly from the peroxo-modified titanate

nanotubes by the dehydration and morphological development as mentioned earlier and shown in

Figure 2f.

Figure 5. Transmission electron microscopy (TEM) images of (a) as-synthesized PTNTs and (b–e)annealed PTNTs at 200, 300, 400, and 500 ◦C for 1 h, respectively. (f) Variation in the interlayer distancefor annealing temperatures taken from 30 zones, respectively.

Figure 6 shows the HR-TEM images and the FFT diffraction pattern of the annealed PTNTs at 400and 500 ◦C. The d-values of the lattice fringes originated from diffraction spots which were calculatedfrom the FFT analysis of the TEM images; they were compared with the PDF card information forhydrogen titanate (# 00-047-0124) and anatase (# 00-021-1272). In the case of the PTNTs sample annealedat 400 ◦C, as shown in Figure 6a, the calculated diffraction spots for one region was estimated to bethe 200 plane of the hydrogen titanate with d-value of 0.7 nm. On the other hand, a lattice fringewith a d-value of 0.37 nm was also observed in the same sample (enlarged image and an FFT patternin Figure 6a), which corresponds to the 101 plane of the anatase, in accordance with XRD results,as shown in Figure 4. As was evident from the XRD analysis results, the annealed PTNTs at 400 ◦Cpossess low crystallinity; the lattice fringes and diffraction spots assigned to only the main 101 planeof anatase were expected to be observable, while other spots (e.g., corresponding to 011, 112) are notexpected to be observed.



Figure 6. TEM images and fast Fourier transform (FFT) diffraction patterns (T = Titanate, A = Anatase)

of annealed PTNT at (a) 400 and (b) 500 °C for 1 h, respectively.

3.5. Bonding Characteristics

Raman spectroscopy and FT-IR analysis were carried out to investigate the effect of annealing

temperature on the structures. Figure 7a shows the Raman spectra of structures annealed at various

temperatures from 200 to 500 °C for 1 h. The Raman pattern of as-synthesized PTNTs showed a

hydrogen titanate structure [46]. The peaks assigned to Ti–O, Ti–O–Ti, and Ti–O–H were observed at

276, 444, and 700 cm−1, respectively. After annealing at 500 °C, the intensity of peaks assigned to Ti–

O and Ti–O–Ti of hydrogen titanate structure decreased, and several new peaks surfaced at 135, 395,

510, and 650 cm−1. These peaks were assigned to Eg, B1g, A1g, and Eg of anatase. The Raman spectrum

of the PTNTs annealed at 500 °C mainly consisted of the pattern of anatase, while weak peaks of

hydrogen titanate structures were also confirmed.

The annealing effect on the PTNTs was further examined using a local structure-determining

probe, XAFS, as shown in Figure S1. Figure S1a shows the Ti K-edge XANES spectra of the as-

synthesized PTNTs and annealed PTNTs at various temperatures with anatase-type TiO2 as the

reference sample, which demonstrate structural differences in the local structure and the electronic

state of Ti4+ ions. Figure 7b shows the FT-IR spectra of samples obtained at the various annealing

temperatures. These spectra were quite similar and were characterized by a broad and strong band

at 3400 cm−1, which can be assigned to OH groups [47] as well as to the H2O molecule. The existence

of this peak indicates the hydroxyl group and water molecules existed in the structure or in the

interlayer space between the crystal layers. The peak intensity decreased with an increase in the

annealing temperature, which led to the evaporation of water molecules and the elimination of

hydroxyl groups by a dehydration reaction. This is also supported by the findings from the TG-DTA

analysis shown in Figure 1. The presence of water molecules in the sample was also identified by the

peak located at 1630 cm−1, which is assigned to the H–O–H deformation mode (H–O–H). Additionally,

in all samples, a weak peak assigned to peroxo bond (–O–O–) was observed at approximately 915

cm−1. The intensity of the peak was slightly reduced by the heat treatment but the peak was still

present even for the sample annealed at 500 °C. This sample mainly consists of anatase crystal as

mentioned earlier; however, it still contains hydrogen titanate. Thus it could be inferred that the

Figure 6. TEM images and fast Fourier transform (FFT) diffraction patterns (T = Titanate, A = Anatase)of annealed PTNT at (a) 400 and (b) 500 ◦C for 1 h, respectively.

At an annealing temperature of 500 ◦C, as shown in Figure 6b, two kinds of crystalline phaseswere also identified. The FFT diffraction spots were estimated as (011), (101), and (112) planes of theanatase as well as (200) plane of the hydrogen titanate. The enlarged image clearly shows the latticefringes of 0.65, 0.37, 0.37, and 0.24 nm, which are assigned to 200 of hydrogen titanate and 101, 011,and 112 of anatase, respectively. These results are consistent with the results of the in situ XRD asmentioned earlier in Figure 4. As earlier discussed, the sample annealed at 500 ◦C exhibited a rod-likemorphology with a width of 20–50 nm (see square dotted frames in Figure 2e). This rod-like PTNTscontained the two crystalline structures of hydrogen titanate and anatase. This analysis reveals thatthe 101 plane of anatase is parallel to the 200 plane of hydrogen titanate, and thus it is believed that thecrystallites of anatase were converted directly from the peroxo-modified titanate nanotubes by thedehydration and morphological development as mentioned earlier and shown in Figure 2f.

3.5. Bonding Characteristics

Raman spectroscopy and FT-IR analysis were carried out to investigate the effect of annealingtemperature on the structures. Figure 7a shows the Raman spectra of structures annealed at varioustemperatures from 200 to 500 ◦C for 1 h. The Raman pattern of as-synthesized PTNTs showed ahydrogen titanate structure [46]. The peaks assigned to Ti–O, Ti–O–Ti, and Ti–O–H were observedat 276, 444, and 700 cm−1, respectively. After annealing at 500 ◦C, the intensity of peaks assigned toTi–O and Ti–O–Ti of hydrogen titanate structure decreased, and several new peaks surfaced at 135,395, 510, and 650 cm−1. These peaks were assigned to Eg, B1g, A1g, and Eg of anatase. The Ramanspectrum of the PTNTs annealed at 500 ◦C mainly consisted of the pattern of anatase, while weakpeaks of hydrogen titanate structures were also confirmed.



peroxo bond found in the FT-IR spectrum might still exist in the PTNTs samples annealed at 500 °C.

In addition, it can also be confirmed by the X-ray photoelectron spectroscopy (XPS) analysis. The O1s

XPS spectrum of as-synthesized PTNTs had Ti–O–O (peroxo-) bond at 533.1 eV [24]. As can be seen

in Figure S3, this peroxo-bond still existed at 533.4 eV in the PTNTs samples annealed at 500 °C with

slightly decreased intensity, implying the thermal stability of the peroxo-bond in the present PTNTs.

Figure 7. (a) Raman spectra and (b) Fourier transform infrared (FT-IR) spectra of the as-synthesized

PTNTs and annealed PTNTs at different temperatures.

3.6. Optical Properties of Materials

The effects of annealing temperature on the optical properties and electrical band gap of the

samples were studied using UV-Vis spectroscopy analysis. Figure 8 shows the reflectivity of the

samples annealed at various temperatures. The UV-Vis spectrum of the as-synthesized PTNTs clearly

indicates that the absorption edge located in the visible light regions is largely red-shifted compared

with common anatase TiO2. This corresponds to the fact that the as-synthesized PTNTs are yellow.

As the annealing temperature increases, the reflectivity of the incident light in the wavelength range

of 350 to 500 nm increases. An increase in reflectivity is expected to reduce the absorption of light due

to an increase in bandgap energy. The inset in Figure 8 show the Tauc plot for determining the optical

band gap of samples calculated by the Kubelka–Munk model. The bandgap energy of the as-

synthesized PTNTs was 2.55 eV, and the value increased to 3.09 eV at an annealing temperature of

500 °C. The variation of the bandgap energy was related to the presence of the peroxo bond and

crystal transformation. Recently, several research groups [11,24] have shown that the bandgap energy

of TiO2 and related compounds can be reduced due to the presence of the peroxo bond in titanate

structures that increase the valance band level. However, the bandgap energy was increased by

annealing, as shown in Figure 8, which might be due to the decomposition of the peroxo bond.

Savinkina et al. [48] reported that the peroxo bond is stable at room temperature in the air but it can

be destroyed by annealing above of 200 °C, and this phenomenon was also observed in this study as

mentioned above.

However, for the 200 °C annealed sample in the present study, two kinds of optical absorption

edges can be seen in the reflectance curve around 360 nm and 420 nm. These can also be found as the

two linear regions in the Tauc plot, and correspond to the band gap energy of 2.92 eV and 2.60 eV,

respectively. The latter value is closed to that of as-synthesized PTNT (2.55 eV) that contains –O–O–

bonding, while the former one is more likely to those of annealed at higher temperature (above 300

°C). By considering these facts, we speculated that the sample annealed at 200 °C may contain two

kinds of phases; one is similar to as-synthesized PTNT and the other was more likely to anatase phase.

Therefore, it is considered that the crystalline phase transformation, in another words nucleation of

anatase-based crystal, might have started around 200 °C, and thus two kinds of optical nature might

be seen in the sample annealed at this temperature.

Figure 7. (a) Raman spectra and (b) Fourier transform infrared (FT-IR) spectra of the as-synthesizedPTNTs and annealed PTNTs at different temperatures.

The annealing effect on the PTNTs was further examined using a local structure-determining probe,XAFS, as shown in Figure S1. Figure S1a shows the Ti K-edge XANES spectra of the as-synthesizedPTNTs and annealed PTNTs at various temperatures with anatase-type TiO2 as the reference sample,which demonstrate structural differences in the local structure and the electronic state of Ti4+ ions.Figure 7b shows the FT-IR spectra of samples obtained at the various annealing temperatures.These spectra were quite similar and were characterized by a broad and strong band at 3400 cm−1,which can be assigned to OH groups [47] as well as to the H2O molecule. The existence of thispeak indicates the hydroxyl group and water molecules existed in the structure or in the interlayerspace between the crystal layers. The peak intensity decreased with an increase in the annealingtemperature, which led to the evaporation of water molecules and the elimination of hydroxyl groupsby a dehydration reaction. This is also supported by the findings from the TG-DTA analysis shown inFigure 1. The presence of water molecules in the sample was also identified by the peak located at1630 cm−1, which is assigned to the H–O–H deformation mode (δH–O–H). Additionally, in all samples,a weak peak assigned to peroxo bond (–O–O–) was observed at approximately 915 cm−1. The intensityof the peak was slightly reduced by the heat treatment but the peak was still present even for the sampleannealed at 500 ◦C. This sample mainly consists of anatase crystal as mentioned earlier; however, it stillcontains hydrogen titanate. Thus it could be inferred that the peroxo bond found in the FT-IR spectrummight still exist in the PTNTs samples annealed at 500 ◦C. In addition, it can also be confirmed by theX-ray photoelectron spectroscopy (XPS) analysis. The O1s XPS spectrum of as-synthesized PTNTs hadTi–O–O (peroxo-) bond at 533.1 eV [24]. As can be seen in Figure S3, this peroxo-bond still existedat 533.4 eV in the PTNTs samples annealed at 500 ◦C with slightly decreased intensity, implying thethermal stability of the peroxo-bond in the present PTNTs.

3.6. Optical Properties of Materials

The effects of annealing temperature on the optical properties and electrical band gap of thesamples were studied using UV-Vis spectroscopy analysis. Figure 8 shows the reflectivity of thesamples annealed at various temperatures. The UV-Vis spectrum of the as-synthesized PTNTs clearlyindicates that the absorption edge located in the visible light regions is largely red-shifted comparedwith common anatase TiO2. This corresponds to the fact that the as-synthesized PTNTs are yellow.As the annealing temperature increases, the reflectivity of the incident light in the wavelength rangeof 350 to 500 nm increases. An increase in reflectivity is expected to reduce the absorption of lightdue to an increase in bandgap energy. The inset in Figure 8 show the Tauc plot for determining theoptical band gap of samples calculated by the Kubelka–Munk model. The bandgap energy of theas-synthesized PTNTs was 2.55 eV, and the value increased to 3.09 eV at an annealing temperature of


500 ◦C. The variation of the bandgap energy was related to the presence of the peroxo bond and crystaltransformation. Recently, several research groups [11,24] have shown that the bandgap energy of TiO2and related compounds can be reduced due to the presence of the peroxo bond in titanate structuresthat increase the valance band level. However, the bandgap energy was increased by annealing, asshown in Figure 8, which might be due to the decomposition of the peroxo bond. Savinkina et al. [48]reported that the peroxo bond is stable at room temperature in the air but it can be destroyed byannealing above of 200 ◦C, and this phenomenon was also observed in this study as mentioned above.


According to the above mentioned discussion, it has been established that annealing caused the

decomposition of peroxo bond from PTNTs and it is expected to result in a decrease in the valance

band level due to the increase in the titanium electron density. The XRD results showed that the

hydrogen titanate crystal of PTNTs transforms into an anatase crystal structure by annealing at a

temperature of 500 °C (Figure 4). At the same time, the TEM observation and Raman spectrum

indicated that an anatase phase in a sample was formed at 500 °C with partial hydrogen titanate

structures (Figure 6 and Figure 7). The reflectance spectrum of the sample annealed at 500 °C showed

a lower reflectivity in the visible light range compared to pure anatase. Furthermore, the bandgap

was calculated to be 3.09 eV which is lower than that of anatase (3.2 eV). By combining the reflectance

spectrum with FT-IR analysis (Figure 7), although the peroxo bond was affected by annealing, the

peroxo bond that exists in the crystal of PTNTs had high stability.

Figure 8. Reflectance spectra of PTNTs annealed at different temperatures. (Inset) Plots of (hv)1/2 vs.

energy for the annealed PTNTs at different temperatures.

3.7. Formation Mechanism of Peroxo-Modified Anatase Crystal

When the crystallographic structure is considered, the H2Ti2O5·H2O (or expressed also as

H2Ti2O4(OH)2) structure, which is hydrogen di-titanate, has been suggested to comprise of two-

dimensional layers in which TiO6 octahedra are combined through edge sharing [34], as shown in

Figure 9. The lattice is orthorhombic, and the TiO6 layers were laminated with an alternating

interlayer cation (H+) along the [100] direction [49]. In the case of the nanotube structure of layered

hydrogen titanate, Zhang et al. [50] reported that these layers scroll the [010] direction with the tube

axis pointing towards the [001] direction. The configuration of the titanate layer, as shown in Figure

9, with projection along the [001] direction of the titanate, is reported to be similar to the principal

unit layer of the anatase projected along the [101] direction [49]. It is hypothesized that, upon

annealing, the titanate layers shrink locally, by reducing the interlayer distance and transforming into

the anatase crystal [47,49]. Our results as well as those of others [47,49] have shown that local

shrinkage of the titanate layers on the hydrogen titanate structure to form anatase crystal structure is

possible.

Figure 8. Reflectance spectra of PTNTs annealed at different temperatures. (Inset) Plots of (αhv)1/2 vs.energy for the annealed PTNTs at different temperatures.

However, for the 200 ◦C annealed sample in the present study, two kinds of optical absorptionedges can be seen in the reflectance curve around 360 nm and 420 nm. These can also be found asthe two linear regions in the Tauc plot, and correspond to the band gap energy of 2.92 eV and 2.60 eV,respectively. The latter value is closed to that of as-synthesized PTNT (2.55 eV) that contains –O–O–bonding, while the former one is more likely to those of annealed at higher temperature (above 300 ◦C).By considering these facts, we speculated that the sample annealed at 200 ◦C may contain two kindsof phases; one is similar to as-synthesized PTNT and the other was more likely to anatase phase.Therefore, it is considered that the crystalline phase transformation, in another words nucleation ofanatase-based crystal, might have started around 200 ◦C, and thus two kinds of optical nature mightbe seen in the sample annealed at this temperature.

According to the above mentioned discussion, it has been established that annealing caused thedecomposition of peroxo bond from PTNTs and it is expected to result in a decrease in the valanceband level due to the increase in the titanium electron density. The XRD results showed that thehydrogen titanate crystal of PTNTs transforms into an anatase crystal structure by annealing at atemperature of 500 ◦C (Figure 4). At the same time, the TEM observation and Raman spectrumindicated that an anatase phase in a sample was formed at 500 ◦C with partial hydrogen titanatestructures (Figures 6 and 7). The reflectance spectrum of the sample annealed at 500 ◦C showed alower reflectivity in the visible light range compared to pure anatase. Furthermore, the bandgap wascalculated to be 3.09 eV which is lower than that of anatase (3.2 eV). By combining the reflectancespectrum with FT-IR analysis (Figure 7), although the peroxo bond was affected by annealing,the peroxo bond that exists in the crystal of PTNTs had high stability.


3.7. Formation Mechanism of Peroxo-Modified Anatase Crystal

When the crystallographic structure is considered, the H2Ti2O5·H2O (or expressed also asH2Ti2O4(OH)2) structure, which is hydrogen di-titanate, has been suggested to comprise oftwo-dimensional layers in which TiO6 octahedra are combined through edge sharing [34], as shown inFigure 9. The lattice is orthorhombic, and the TiO6 layers were laminated with an alternating interlayercation (H+) along the [100] direction [49]. In the case of the nanotube structure of layered hydrogentitanate, Zhang et al. [50] reported that these layers scroll the [010] direction with the tube axis pointingtowards the [001] direction. The configuration of the titanate layer, as shown in Figure 9, with projectionalong the [001] direction of the titanate, is reported to be similar to the principal unit layer of the anataseprojected along the [101] direction [49]. It is hypothesized that, upon annealing, the titanate layersshrink locally, by reducing the interlayer distance and transforming into the anatase crystal [47,49].Our results as well as those of others [47,49] have shown that local shrinkage of the titanate layers onthe hydrogen titanate structure to form anatase crystal structure is possible.Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 17

Figure 9. Hypothetical schematic view of a hydrogen titanate crystal.

Figure 10 illustrates the hypothesis system for the transformation from titanate nanotubes to

titania nanoplates, mimicking the lattice fringes of titanate or titania shown in the above TEM results.

We observed that water, hydroxyl (–OH), and/or peroxo (–O–O–) groups were removed by

dehydration reaction, which resulted from annealing that occurred in the interlayer of hydrogen

titanate structure, thereby reducing the distance between that titanate layers (also see Figure 9).

During this process, tubular structures with hydrogen titanate were transformed into sheet-like

structures by annealing because of the removal of water from the interlayer. As discussed in the

previous section and shown in Figure 2, this process may govern the morphological change by the

formation of cleavage in the tubular titanate along the tube axis (crystallographic variation is

illustrated in Figure 10), which results in the formation of stacked titanate nanosheets during heat

treatment. Anatase crystal was then formed simultaneously along the layered hydrogen titanate by

annealing, thereby titania nanoplate or rod structure that partially contained hydrogen titanate

crystal was formed. Several research groups [30–32,34] have reported the annealing effects on TNTs,

in which almost all TNTs transform into a spherical morphology with a clear anatase structure after

annealing at 500 °C.

Figure 10. Hypothetical scheme for the transformation of titanate nanotubes to titania nanoplates. The

structure models presented are the projections along the nanotube. The transformed structure shown

is the anatase phase.

However, these previous reports are quite different from our present study. The difference in

morphological change by annealing might be attributed to the peroxo groups in the interlayer of

PTNTs. Our bottom-up process using the peroxo-titanium complex ion precursor was different from

traditional methods [4,16–18] for the synthesis of nanotubular titania. During this process, the special

peroxo-titanium bonds (Ti–O–O–) can be formed directly within the interlayers of the hydrogen

titanate crystal as well as on the surface of materials during crystal formation without any chemical

treatment [24]. Usually, the peroxo groups of TNTs are expected to be expelled by annealing.


Figure 10 illustrates the hypothesis system for the transformation from titanate nanotubes totitania nanoplates, mimicking the lattice fringes of titanate or titania shown in the above TEM results.We observed that water, hydroxyl (–OH), and/or peroxo (–O–O–) groups were removed by dehydrationreaction, which resulted from annealing that occurred in the interlayer of hydrogen titanate structure,thereby reducing the distance between that titanate layers (also see Figure 9). During this process,tubular structures with hydrogen titanate were transformed into sheet-like structures by annealingbecause of the removal of water from the interlayer. As discussed in the previous section and shownin Figure 2, this process may govern the morphological change by the formation of cleavage in thetubular titanate along the tube axis (crystallographic variation is illustrated in Figure 10), which resultsin the formation of stacked titanate nanosheets during heat treatment. Anatase crystal was thenformed simultaneously along the layered hydrogen titanate by annealing, thereby titania nanoplateor rod structure that partially contained hydrogen titanate crystal was formed. Several researchgroups [30–32,34] have reported the annealing effects on TNTs, in which almost all TNTs transforminto a spherical morphology with a clear anatase structure after annealing at 500 ◦C.

However, these previous reports are quite different from our present study. The difference inmorphological change by annealing might be attributed to the peroxo groups in the interlayer ofPTNTs. Our bottom-up process using the peroxo-titanium complex ion precursor was different fromtraditional methods [4,16–18] for the synthesis of nanotubular titania. During this process, the specialperoxo-titanium bonds (Ti–O–O–) can be formed directly within the interlayers of the hydrogentitanate crystal as well as on the surface of materials during crystal formation without any chemicaltreatment [24]. Usually, the peroxo groups of TNTs are expected to be expelled by annealing. However,peroxo titanium bonds in the crystal structure of PTNTs could be restricted by the crystal transformation


to anatase because this bond within the interlayer is harder to remove than that on the surface dueto the difference in the heat-transfer distance from the surface to the interior [51]. Peroxo bondingwas confirmed using the FT-IR of the annealed PTNTs. As a result, the peroxo group is thoughtto suppress the shrinkage that would have occurred between layers as a result of heat treatment.The titania nanoplates as the final products were considered to be composed of anatase crystal andlayered hydrogen titanate structure with peroxo bond in their layers (Figure 10). Etacheri et al. [11]reported that oxygen vacancy accelerates the Ti–O breaking and phase transition of titania duringannealing. Although a detailed analysis is required in future studies, the peroxo groups in PTNTsare expected to inhibit the generation of oxygen vacancy due to the release of oxygen by thermaldecomposition. Therefore, PTNTs synthesized from Ti–O–O complex ions are also expected to partiallyremain in the structures due to their high thermal stability. These results indicated that the bottom-upprocess using ion-complex materials, which is an environmentally friendly method, is a unique methodfor constructing crystalline structure and morphology tunable nanostructured titanate and it has thepotential to enhance the thermal stability of the nanomaterials.



Figure 10 illustrates the hypothesis system for the transformation from titanate nanotubes to

titania nanoplates, mimicking the lattice fringes of titanate or titania shown in the above TEM results.

We observed that water, hydroxyl (–OH), and/or peroxo (–O–O–) groups were removed by

dehydration reaction, which resulted from annealing that occurred in the interlayer of hydrogen

titanate structure, thereby reducing the distance between that titanate layers (also see Figure 9).

During this process, tubular structures with hydrogen titanate were transformed into sheet-like

structures by annealing because of the removal of water from the interlayer. As discussed in the

previous section and shown in Figure 2, this process may govern the morphological change by the

formation of cleavage in the tubular titanate along the tube axis (crystallographic variation is

illustrated in Figure 10), which results in the formation of stacked titanate nanosheets during heat

treatment. Anatase crystal was then formed simultaneously along the layered hydrogen titanate by

annealing, thereby titania nanoplate or rod structure that partially contained hydrogen titanate

crystal was formed. Several research groups [30–32,34] have reported the annealing effects on TNTs,

in which almost all TNTs transform into a spherical morphology with a clear anatase structure after

annealing at 500 °C.

Figure 10. Hypothetical scheme for the transformation of titanate nanotubes to titania nanoplates. The

structure models presented are the projections along the nanotube. The transformed structure shown

is the anatase phase.

However, these previous reports are quite different from our present study. The difference in

morphological change by annealing might be attributed to the peroxo groups in the interlayer of

PTNTs. Our bottom-up process using the peroxo-titanium complex ion precursor was different from

traditional methods [4,16–18] for the synthesis of nanotubular titania. During this process, the special

peroxo-titanium bonds (Ti–O–O–) can be formed directly within the interlayers of the hydrogen

titanate crystal as well as on the surface of materials during crystal formation without any chemical

treatment [24]. Usually, the peroxo groups of TNTs are expected to be expelled by annealing.

Figure 10. Hypothetical scheme for the transformation of titanate nanotubes to titania nanoplates.The structure models presented are the projections along the nanotube. The transformed structureshown is the anatase phase.

4. Conclusions

In this study, the thermal stability of peroxo titanate nanotubes (PTNTs) synthesized by theperoxo titanium complex ion was evaluated. The as-synthesized structure was a tubular structureof hydrogen titanate H2Ti2O5·H2O that contained peroxo groups within the interlayers of the crystalstructure. Dehydration and thermal decomposition by annealing were carried out in two steps:(I) Dehydration reaction of a water molecule on the surface at an annealing temperature range below143 ◦C, (II) Dehydration of water and –OH groups in the crystal and transition into anatase at anannealing temperature range above 143 ◦C. In step I, the water loss occurred by evaporation, and instep II, dehydration reaction resulted in interstructural agglomeration and shrinkage. In addition,the transition of the interlayer of the crystal from hydrogen titanate to anatase occurred, and atthe same time, the tubular structure was dismantled due to the formation of a crystal structure.The as-synthesized PTNTs had the highest surface area and it continued to decrease as the annealingtemperature increased. Moreover, as the annealing temperature increased to 360 ◦C, anatase crystalbegan to form on the structure and the morphology evolved from nanotube to nanoplate-likestructure along with the formation of a long axis of the nanotube structure through the transientmorphology with stacked leaf-like nanosheets at an intermediate temperature that ranged from 200 ◦Cto approximately 400 ◦C. The crystal structure of the nanoplate heated at 500 ◦C was a mixture ofhydrogen titanate and anatase. Optical studies showed the extension in bandgap energies uponannealing. In addition, the results from our experiment showed that the presence of peroxo bonds


within the interlayer space, which is confirmed to remain even after heat treatment at 500 ◦C in themixed titanate crystal, was favors its thermal stability more than when the bond exists on the surface.Although the crystallographic properties changed with an increase in temperature above 143 ◦C,the relationship between the structure, morphology, optical properties of peroxo-titanate nanotubes,and the annealing temperature became clear from this study. Therefore, our findings are quite helpfulfor the structural design of titanate nanomaterials for various applications based on the synergy oftheir unique low-dimensional nanostructures and physical-chemical properties. Moreover, our visiblelight-activated titanate nanostructures with the high thermal stability of peroxo bond can be consideredas promising material for photocatalytic and/or photovoltaic applications.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/7/1331/s1:Figure S1: X-ray absorption fine structure (XAFS) of anatase, as-synthesized PTNTs and annealed PTNTs at 200,300, 400, and 500 ◦C for 1 h, respectively: (a) X-ray absorption near edge structure (XANES) spectra, (b) enlargedpre-edge regions, (c) enlarged white line regions, and (d) Fourier-transformed extended X-ray absorption finestructure (EXAFS) spectra. The radial distribution functions were not corrected for phase shift.

Author Contributions: Conceptualization, H.P. and T.S.; Formal analysis, H.P., T.G. and S.C.; Funding acquisition,T.S.; Investigation, H.P., T.G. and S.C.; Methodology, H.P. and T.S.; Project administration, T.S.; Resources, S.W.L.;Supervision, T.S.; Writing – original draft, H.P.; Writing – review and editing, T.G., M.K. and T.S. All authors haveread and agreed to the published version of the manuscript.

Funding: This research was funded by the Japan Society for the Promotion of Science (JSPS) under the Grants-in-Aidfor Scientific Research (S) (grant number 15H05715), and supported by the “Dynamic Alliance for Open InnovationBridging Human, Environment and Materials” in “Network Joint Research Center for Materials and Devices”(MEXT, Japan).

Acknowledgments: TG-DTA and FT-IR analysis were performed at the Comprehensive Analysis Center, ISIR,Osaka University, Japan. The authors are grateful to T. Takehara (Osaka Univ., Japan) for his support in TG-DTAanalysis. Raman investigation of samples was performed at the Sunmoon University, Republic of Korea. The XAFSexperiments were performed at Kyushu University Beamline (SAGA-LS/BL06) with the proposal of No. 2019IIK010.The authors are grateful to T. Sugiyama (Kyushu Univ., Japan) and T. Ishioka (Kyushu Univ., Japan) for theirsupport in XAFS measurement. The authors are grateful to Prof. S. Seino for his support with TEM observations.

Conflicts of Interest: The authors declare no conflict of interest.

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